Page 415 - Handbook of Adhesives and Sealants

360 Chapter Ten

sistency. B-staged epoxy resins are formulated using curing agents
such as aliphatic amines and aromatic polyamines along with solid
forms of epoxy resins.
Secondary ingredients in epoxy adhesives include reactive diluents
to adjust viscosity; mineral fillers to lower cost, adjust viscosity, or
modify coefficient of thermal expansion; and fibrous fillers to improve
thixotropy and cohesive strength. Epoxy resins are often modified with
other resins to enhance certain properties that are necessary for the
application. Often these modifications take the form of additions of
elastomeric resins to improve toughness or peel strength.

10.3.1.1 Epoxy resins. The ‘‘signature’’ of an epoxy molecule is the
three membered ring containing oxygen.

— O —
R — CH — CH 2

Most epoxy formulations are centered around the use of the diglycidyl
ether of bisphenol A (DGEBA) as the base epoxy resin (Fig. 10.2). The
DGEBA type of epoxy resin used in most adhesive formulations is
derived from the reaction of bisphenol A and epichlorohydrin. When
the repeating unit, n, equals 0.1 to 1.0, the epoxy resins are normally
viscous liquids. When n exceeds 2.0, the resins are solids and need to
be melted or dissolved in solvent prior to processing. Another epoxy
resin that is often used is diglycidyl ether of Bisphenol F (DGEBF).
This resin has a lower viscosity that allows greater possibilities in
formulation. There are many other epoxy resins available to the ad-
hesive formulator. Some of these resins have multiple functionality.
Generally, a higher functionality improves heat and environmental
resistance, but it may result in a more brittle product.

Figure 10.2 General structure of DGEBA (diglycidyl ether of bisphenol A) epoxy resin
molecule.